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The Re attack ends up being favored 4:1, due to Felkin-Anh interactions. Felkin-Ahn Model Chelate Model Diastereoselective HiI am Jayanta Dey (JRF,Chemistry). In this reaction, the Re attack is favored for the reason that it pushes the oxygen forward and there is no eclipsing. Addition on a carbonyl tends to follow this trend in any case it is the counter-effects which make the other isomers appear in general. This is another way to control, or at least explain, stereochemistry. They come together so that the orbitals of the newly formed sp3 carbon do not eclipse with the already existingīonds, or so that the largest substituent on the neighboring carbon is anti to An important aspect of these models is the. A more useful perspective for this analysis is a model represented by 298, which suggests that approach via path b is encumbered by the axial hydrogens on the bottom face of the molecule. The ratio of this reaction was 33:67.įelkin-Anh is based on a reaction in which the metal and nucleophilic group both interact with the ends of the carbonyl bond simultaneously. The Cram model or the Felkin-Anh model predict path b as the major pathway (to give 300), which is incorrect. Why? The group that hangs off behind the chelating group in the reactant hinders many of the Grignard reagents' attempts to attack from the Si side. Tetrahedron Asym., 2001, 12,Īnd when the reaction shown below was done, the Si addition isomer was greatly favored. The Felkin Anh Model L M L M Nu OH M u HO O R 2) H+ 1) M Nu Entry FA:C Felkin-Anh (FA)Cram-Chelat (CC) M Nu R R S S S L M S R 19 Ph p-Tol 17 : 83 20 Ph 02 : 98 21 Ph 04 : 96 p-Tol 22 Ph Ph 12 : 88 23 Ph h 34 : 66 13 NBn2 Me 95 : 05 14 NBn2 Me 06 : 94 24 Ph 23 : 77 15 SMe Et Ph Li(sBu)3BH 99 : 01 17 CO2Me Me Ph 73 : 27 18 M e Ph 03 : 97 16 SMe. In fact, one of the compounds in this article:Ĭarda, M. The nucleophile can only attack from the 'bottom' of our view. Chelation is best for telling us what side will be less hindered for attack because the atom keeps its stereochemistry in the rigid chelate form. These ideas are important because they can help determine the stereochemistry of additions. Felkin-Anh, so far as we can tell, is just a transition state in which the nucleophile and metal interact with the two ends of the carbonyl at the same time. Stereoselectivity in carbonyl reactions is often handled in two ways: with explanations based on the formation of a chelate (a claw-like chemical formation, figure 1), or with an appeal to a Felkin-Anh transition state (figure 2).Ī chelate is basically a not-really there 5-membered ring in which a metal on a Grignard reagent (that's a Magnesium alkylate) interacts with the double-bonded oxygen and a fairly strong nucleophile three bonds away. Remarkably, if this ketone is reduced by the bulky borane reagent, disiamylborane (C 5 H 11) 2 BH, the diastereoselectivity is reversed, with isomer B being formed in 82 de. Stereoselectivity in additions to carbonyl The Felkin-Ahn model shown to the right of the equation correctly predicts the diastereoselectivity of the reduction in favor of isomer A.